The present invention relates to power control, and in particular, to circuits and methods for controlling power in a battery operated system.
Batteries have long been used as a source of power for a variety of electronic systems. Batteries provide energy in the form of electric currents and voltages that allow circuits to operate. However, the amount of energy stored in a battery is limited, and batteries lose power when the electronic devices are in use. When a battery's energy supply becomes depleted, the battery's voltage will start to fall from its rated voltage. When the voltage on the battery falls below a threshold, the electronic device relying on the battery for power will no longer operate properly. Such thresholds will be different for different types of electronic devices.
Additionally, different electronic systems may draw different amounts of current on one or more power supply input terminals. Moreover, the current drawn in any given system, or any portion of the system, may vary over time as the system's electronics enter different modes of operation. For example, an electronic system that includes an internal magnetic drive (i.e., a hard drive) may draw more current, and hence more power, when powering up the circuits for interfacing with the hard drive (e.g., motor driver circuitry). Typically, the circuits in an electronic system will not operate properly unless the system receives one or more power supply voltages at or above one or more corresponding thresholds. For example, some systems may require one power supply voltage of at least 3.5 volts to operate properly. When the power supply voltage is below a minimum power supply threshold voltage, the system may not be able to power up.
One problem associated with battery operated systems results from limits on external power sources supplying voltages and currents. Many sources of power have limits on the amount of current the power source can provide. In some situations, a user may desire to both operate the system and charge the battery at the same time. However, limits on the available current may cause battery charging to be suboptimal. For example, if an external power source has a maximum available current, some modes of system operation may approach or exceed this maximum, causing the voltage on the external power source to drop. Such a voltage drop may cause problems with the external system. Additionally, it may be desirable to efficiently charge the battery and operate the system at the same time. However, as the system draws more current, there is less current for use in charging the battery. It is generally desirable to improve battery charging while the system is drawing current, and in particular, it is desirable to improve battery charging when the system draws current close to the current limit of the external power source.
Another problem associated with battery operated systems is that some external power sources may have different current modes, and may require data communication with the battery operated system to move from one current mode to another. For example, USB Host systems are capable of providing voltage (e.g., 5 volts) and current that may be used for powering a system and/or charging the systems battery. However, USB systems may initially be configured in a first current mode that only provides up to 100 mA of current. A battery operated system may communicate with the USB Host in a process sometimes referred to as “enumeration,” and the USB Host may then change to another current mode capable of providing up to 500 mA of current. However, 100 mA may not be sufficient current to power the battery operated system. Moreover, if the voltage on the system battery is depleted below a minimum threshold for proper system operation (e.g., below 3.5 volts in a 5 volt system), then the system will not be able to power up. Accordingly, the system will not be able to communicate with the USB Host. Consequently, even though the USB Host may be capable of delivering 5 volts and 500 mA of current to the system, which may be used for charging the battery and/or powering up other system electronics, the USB Host may never transition into a higher current mode because the battery operated system does not have the initial power to turn on and communicate with the USB Host.
Thus, there is a need for improved power control in a battery operated system. The present invention solves these and other problems by providing circuits and methods for controlling power in a battery operated system.